In general, a manual motor starter (so called MMS as abbreviated) serves as a switching device which has a function for protecting a motor by interrupting power supply to the motor upon generation of a fault current in a section for starting or stopping the motor, such as an electric shortage, a ground fault and an electric phase deficiency.
FIG. 1 is a schematic cross-sectional view of a conventional manual motor starter to which an arc extinguishing device is applied.
A conventional circuit breaker (1) for manual motor starter (hereinafter referred to as MMS) largely has a structure in which an upper frame (3) and a lower frame (5) are coupled. A detector (10) usually detects a failure current inputted from a power source terminal (12). An open/close mechanism (20) opens and closes the circuit breaker (1) in response to a detection signal from the detector (10). The open/close operation of the circuit breaker is implemented in such a manner that a movable contact (36) of a movable contact bar (35) is separated from a stationary contact (33) of a stationary contact bar (32) by the operation of the open/close mechanism (20) to prevent a failure current from flowing to a load, i.e., a motor via a motor-side terminal (14), thereby protecting the motor.
A high pressure and high temperature arc is generated between the movable and stationary contacts (33, 36) when the movable contact (36) of the movable contact bar (35) is separated from the stationary contact (33) of the stationary contact bar (32). Unless the arc is swiftly extinguished and discharged outside, the movable and stationary contacts (33, 36) may be damaged by the high pressure and high temperature arc, and in worst case, the arc may result in failure of interruption of the fault current which in turn damages the product. Therefore, it is essential that the arc be swiftly extinguished and discharged outside of the circuit breaker (1)
Now, structure and operation of a conventional arc extinguishing device (40) in the circuit breaker (1) will be described in greater detail with reference to FIGS. 2, 3, 4 and 5.
Referring to FIG. 2, an arc chamber (50) of a conventional arc extinguishing device (40) includes a plurality of magnetic plates (52) and a side wall (54) in which the plurality of magnetic plates (52) are packaged. Each magnetic plate (52) may have a U-shaped member opened in the direction of arc generation, and induce the arc (a) in response to the electromagnetic force.
When a fault current is introduced, the movable contact (36) is separated from the stationary contact (33) by the open/close mechanism (20) of FIG. 1 to generate a high pressure and high temperature arc. Although the movable and stationary contacts (33, 36) are discrete, a phenomenon of a current flowing by the arc (a) is generated, such that the fault current is not completely interrupted. The arc (a) is induced into the arc chamber (50) by the electromagnetic force of the plurality of magnetic plates (52). At this time, the arc (a) is further sped up in movement along the conductive stationary contact bar (32) and a lower plate (60). In the course of this process, the arc (a) is extinguished by cathode effect and cooling effect of the magnetic plates (52) to finally interrupt the flow of the fault current and is discharged via an arc outlet (55) formed at the side wall (54).
However, because the short-circuit current introduced in the circuit breaker for MMS is large in most cases, a strong arc is generated and induced between the movable and stationary contacts (33, 36). As a result, the arc (a) generated from within the arc chamber (50) is not completely extinguished to generate a residual arc (b) connecting a distal end of the stationary contact bar (32) and a distal end of the lower plate (60). The residual arc (b) allows the fault current to flow, thereby resulting in a problem of delaying the interruption time.
Another problem is that distal ends of the stationary contact bar (32) and the lower plate (60) are melted down due to the residual arc (b) generated from the distal ends of the stationary contact bar (32) and the lower plate (60), or generates a thermal deformation bending toward the magnetic plate. The thermal deformation markedly degrades the performance of arc extinguishment, thereby leading to decrease of short-circuit interruption capacity of the circuit breaker.
In order to alleviate the problems caused by the residual arc (b), an arc extinguishing device (40′) is disclosed in the European Patent Registration No. 1,032,942.
FIG. 4 is a schematic perspective view of an arc chamber (50′) that has improved the conventional arc extinguishing device. The arc extinguishing device (40′) of FIG. 4 is distinguished from the arc chamber (50) of FIG. 2 in that an extension (56) protruded to an upper surface of the side wall (54) disposed with an arc outlet (55) is formed at the improved arc chamber (50′).
Referring to FIG. 5, the extension (56) of the improved arc chamber (50′) is made of insulation material as that of the side wall (54), and is extended to abut on a distal end of the stationary contact bar (32). As a result, the arc (a) generated between the distal end of the stationary contact bar (32) and the distal end of the lower plate (60) can be interrupted.
However, the conventional improved arc chamber (50′) suffers from disadvantages in that the extension (56) has a protrusive shape that tends to be deformed in the course of manufacturing or transportation of the arc chamber (50′), thereby disabling to interrupt the arc generated between the distal end of the stationary contact bar (32) and that of the lower plate (60). Another disadvantage of the conventional improved arc chamber (50′) is that the arc chamber (50′) may be manufactured upside down due to a mistake by assembling workers. In other words, the extension (56) may be positioned at the lower plate (60) to disable to interrupt the arc (a) generated between the distal end of the stationary contact bar (32) and that of the lower plate (60).